This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-195897, filed Jun. 29, 2000; and No. 2001-119664, filed Apr. 18, 2001, the entire contents of both of which are incorporated herein by reference.
The present invention relates generally to a cathode-ray tube apparatus and more particularly to a color cathode-ray tube apparatus capable of improving an oval distortion of a beam spot shape on a peripheral portion of a phosphor screen and stably providing a high image quality.
A currently dominant self-convergence type inline color cathode-ray tube apparatus comprises an inline electron gun assembly for emitting three in-line electron beams, which travel on a horizontal plane, and a deflection yoke for generating non-uniform deflection magnetic fields for deflecting the electron beams emitted from the electron gun assembly. The deflection magnetic fields comprise a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection field. As the degree of deflection of the electron beams increases, the deflection magnetic fields will have a stronger action as an equivalent quadrupole lens for vertically focusing the electron beams and horizontally diverging the electron beams.
The distance between the electron gun assembly and the phosphor screen increases as the location of deflected electron beams shifts from a central portion to a peripheral portion of the phosphor screen. Owing to the difference in this distance, while the electron beams are focused at the central portion of the phosphor screen, the electron beams are defocused at the peripheral portion of the phosphor screen.
Accordingly, the beam spot at the peripheral portion of the phosphor screen is optimally focused in the horizontal direction by virtue of mutual cancellation of the diverging action of the deflection magnetic field and the defocusing due to the difference in distance. However, the beam spot at the peripheral portion of the phosphor screen is over-focused in the vertical direction by the addition of the focusing action of the deflection magnetic field and the defocusing due to the difference in distance. Consequently, the beam spot formed on the central portion of the phosphor screen is substantially circular, while the beam spot formed on the peripheral portion of the phosphor screen includes a horizontally elongated high-luminance portion (core) and a vertically elongated low-luminance portion (halo). Because of this, the resolution at the peripheral portion of the phosphor screen considerably deteriorates.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 61-99249 discloses a DAF (Dynamic Astigmatism and Focus) electron gun assembly. This electron gun assembly is characterized in that a third grid, which functions as a focus electrode, comprises a first segment G3-1 and a second segment G3-2. An electron beam passage hole formed at the second segment (G3-2) side surface of the first segment G3-1 has a vertically elongated shape. An electron beam passage hole formed at the first segment (G3-1) side surface of the second segment G3-2 has a horizontally elongated shape. In addition, a dynamic voltage, which is obtained by superimposition of an AC component varying parabolically in accordance with a variation in the degree of deflection of electron beams, is applied to the second segment G3-2.
Thus, in accordance with the deflection of the electron beams, a potential difference occurs between the first segment and the second segment. This potential difference creates a quadrupole lens between the first segment and second segment, which horizontally focus the electron beams and vertically diverges the electron beams. The quadrupole lens compensates a deflection aberration occurring due to the deflection of electron beams. In addition, since the second segment is supplied with the dynamic voltage, the focusing action of the main lens is weakened in accordance with the increase in the deflection amount of the electron beams. Thus, the defocusing due to the aforementioned difference in distance is also corrected.
The electron gun assembly, however, has two problems: 1) as the degree of deflection of electron beams increases, the distance between the electron gun assembly and the phosphor screen increases and the beam spot size increases accordingly, and 2) as the degree of deflection of electron beams increases, the beam spot formed on the phosphor screen is horizontally deformed. Owing to these two problems, the beam spot formed at the peripheral portion of the phosphor screen has an increased average size and a deformed shape.
An explanation will now be given of the phenomenon occurring with this electron gun assembly, in which the beam spot size increases at the peripheral portion of the phosphor screen.
FIGS. 8A and 8B show simplified models for explanation based on only the distance between the electron gun assembly and the phosphor screen, and the power of the main lens. Thus, FIGS. 8A and 8B omit illustration of the quadrupole lens component created by the deflection magnetic fields and the quadrupole lens formed in the electron gun assembly.
The size of the beam spot on the phosphor screen depends on a magnification M expressed by the ratio of a divergence angle xcex1o of an electron beam emitted from an electron beam generating section of the electron gun assembly to an incidence angle xcex1i on the phosphor screen. Thus, the magnification M is given by
M=(divergence angle xcex1o/incidence angle xcex1i).
As is shown in FIG. 8A, in a case where an electron beam is focused on a central portion of the phosphor screen, the electron beam emitted from an object point O at divergence angles xcex1o in both horizontal and vertical directions is focused by a main lens 20 and made incident on the phosphor screen with incidence angles xcex1i(1) in both the horizontal and vertical directions. A magnification M(1) in this case is expressed by
M(1)=xcex1o/xcex1i(1).
As is shown in FIG. 8B, when the electron beam is focused on a peripheral portion of the phosphor screen, the distance between the electron gun assembly and the phosphor screen increases. The electron beam emitted from the object point O at divergence angles xcex1o in both horizontal and vertical directions is focused by the main lens. In the electron gun assembly disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-99249, the focal distance is increased by weakening the focusing power of the main lens. The electron beam focused by the main lens is made incident on the phosphor screen with incidence angles xcex1i(2) in both the horizontal and vertical directions. A magnification M(2) in this case is expressed by
M(2)=xcex1o/xcex1i(2).
Since the distance between the object point O and the main lens is constant, the magnification xcex1i(2) decreases as the distance (focal distance) between the main lens and the phosphor screen increases. Since xcex1i(1) greater than xcex1i(2),
M(1) less than M(2).
When the focal distance is varied by the main lens power, the magnification M increases and the beam spot size on the phosphor screen increases in accordance with the increase in the focal distance. Thus, in the case of the electron gun assembly disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-99249, the average spot size of the beam spot formed on the peripheral portion of the phosphor screen is larger than that of the beam spot formed on the central portion of the phosphor screen.
An explanation will now be given of the phenomenon in which the electron beam spot on the peripheral portion of the phosphor screen is horizontally deformed, using an optical lens model as well. A horizontal magnification Mx of the electron beam and a vertical magnification My of the electron beam are expressed by
Mx (horizontal magnification)=xcex1ox (horizontal divergence angle)/xcex1ix (horizontal incidence angle), and
My (vertical magnification)=xcex1oy (vertical divergence angle)/xcex1iy (vertical incidence angle).
When the electron beam is not deflected, as shown in FIG. 8A, the electron beam emitted from the object point O with the divergence angles xcex1o in the horizontal direction x and vertical direction Y is focused by the main lens 20 with no astigmatism. The beam is then made incident on the phosphor screen with the incidence angles xcex1i (1) in the horizontal direction X and vertical direction Y. In this case, the horizontal magnification Mx is equal to the vertical magnification My, and a circular beam spot is formed.
On the other hand, when the electron beam is deflected, as shown in FIG. 8C, a quadrupole lens component 30 created by the deflection magnetic fields and a quadrupole lens 21 for correcting the lens component 30 are newly added. The electron beam emitted from the object point O with the divergence angles xcex1o in the horizontal direction X and vertical direction Y travels through the quadrupole lens 21, main lens 20 and quadrupole lens component 30 created by the deflection magnetic fields. The beam is thus made incident on the phosphor screen with an incidence angle xcex1ix(3) in the horizontal direction X and an incidence angle xcex1iy(3) in the vertical direction. In this case, a horizontal magnification Mx(3) of the electron beam and a vertical magnification My(3) of the electron beam are expressed by
Mx(3)=xcex1o/xcex1ix(3), and
My(3)=xcex1o/xcex1iy(3).
As is clear from FIG. 8C,
xcex1ix(3) less than xcex1iy(3). Thus, the relationship between the horizontal magnification Mx(3) and vertical magnification My(3) is given by
Mx(3) greater than My(3). Accordingly, the beam spot formed at the peripheral portion of the phosphor screen is horizontally elongated.
This problem occurs because the astigmatism caused by the deflection magnetic fields is compensated by the quadrupole lens located away from the deflection magnetic fields. In order to suppress horizontal elongation of the beam spot on the peripheral portion of the phosphor screen, it is necessary to decrease the distance between the magnetic fields and the quadrupole lens that compensates the astigmatism caused by the deflection magnetic fields.
As has been stated above, in order to enhance the image quality of the cathode-ray tube apparatus, it is imperative that the beam spot have a uniform shape over the entire surface of the phosphor screen. It is thus necessary to simultaneously compensate, as the degree of deflection of electron beams increases, the defocusing due to the increase in distance between the electron gun assembly and the phosphor screen and the astigmatism due to the deflection magnetic fields.
In the typical prior-art electron gun assembly as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-99249, a proper parabolic dynamic voltage is applied to the low-voltage side electrode of the main lens to vary the main lens power, thereby correcting the defocusing. At the same time, by forming a dynamically varying quadrupole lens, the astigmatism due to deflection magnetic fields is corrected.
However, if the beam spot on the central portion of the phosphor screen is made substantially circular, the beam spot shape on the peripheral portion of the phosphor screen would be considerably horizontally elongated and the average size of the beam spot would increase.
The horizontal elongation of the beam spot on the peripheral portion of the phosphor screen occurs for the following reason. If the astigmatism of deflection magnetic fields is to be compensated by the quadrupole lens located on the cathode-side of the main lens, there is a distance between the quadrupole lens component due to the deflection magnetic fields and the quadrupole lens within the electron gun assembly. This distance increases the difference between the horizontal magnification Mx and vertical magnification My. Thus, the beam spot is horizontally elongated.
Besides, since the defocusing occurring when the electron beam is deflected towards the peripheral portion of the phosphor screen is compensated by varying the main lens power, the magnification at the peripheral portion of the phosphor screen becomes greater than that at the central portion of the phosphor screen. As a result, the average size of the beam spot at the peripheral portion of the phosphor screen increases.
The present invention has been made in consideration of the above problems, and its object is to provide a cathode-ray tube apparatus capable of forming a beam spot with a uniform shape over the entire surface of a phosphor screen.
The present invention provides a cathode-ray tube apparatus comprising:
an electron gun assembly having an electron beam generating unit for generating an electron beam, at least one auxiliary lens for prefocusing the electron beam generated from the electron beam generating unit, and a main lens for focusing the electron beam prefocused by the auxiliary lens on a phosphor screen; and
a deflection yoke for generating deflection magnetic fields for horizontally and vertically deflecting the electron beam emitted from the electron gun assembly,
wherein the electron gun assembly comprises a focus electrode, at least one additional electrode and an anode, which are arranged in a direction of travel of the electron beam and constitute the main lens, and also comprises a voltage applying means for applying predetermined voltages to the respective electrodes constituting the main lens,
the voltage applying means applies a constant focus voltage to the focus electrode, a constant anode voltage, which is higher than the focus voltage, to the anode, and a voltage, which is higher than the focus voltage and lower than the anode voltage and varies in accordance with deflection of the electrode beam, to the additional electrode,
the main lens varies such that a vertical focusing power becomes lower than a horizontal focusing power in accordance with an increase in deflection amount of the electron beam, and
the at least one auxiliary lens has a focusing power decreasing in accordance with an increase in deflection amount of the electron beam.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.